docs: def validate_unique_slug(self):
          """
          Ensure slug is unique for this model. This check is aware of publishing
          but is otherwise fairly basic and will need to be customised for
          situations where models with slugs are not in a flat hierarchy etc.
          """
          clashes_qs = type(self).objects.filter(slug=self.slug)
          if self.pk:
              clashes_qs = clashes_qs.exclude(pk=self.pk)
          if isinstance(self, PublishingModel):
              clashes_qs = clashes_qs.filter(
                  publishing_is_draft=self.publishing_is_draft)
          if clashes_qs:
              raise ValidationError(
                  "Slug '%s' clashes with other items: %s"
                  % (self.slug, clashes_qs))